1. Field of the Invention
This invention relates generally to a servo system for a rigid disk drive and more particularly to a servo system utilizing a unique addressing scheme and a special pattern to be utilized before the address and following the burst signals to improve the AGC circuit operation.
2. Description of the Prior Art
In the field of magnetic recording utilized in disk drives where a plurality of tracks are recorded on a rigid magnetic disk, commonly in drives having a small number of disks, embedded servo is utilized to identify the address of the tracks, the sectors within the tracks and for head positioning through a feedback control systems. Typically the tracks are recorded concentrically and within the tracks a plurality of sectors are provided to identify recording areas where user data may be stored. Address information is provided for each track at its origin as well as for each sectors within each track. This track address and sector information is utilized when locating a track where information is to be stored or from where it is to be retrieved, and the address information is also used when moving from track to track under the control of commands from a central processor or host computer.
In the prior art, one approach for providing track address information on a magnetic recording disk is illustrated by U.S. Pat. No. 4,424,543 to Lewis et al. issued Jan. 3, 1984 and entitled "Method and Apparatus for Recording Transducer Positioning Information". In Lewis et al., the tracks on the magnetic recording medium are grouped in two bands of 16 each, with each track within a band being numbered 0-15. Identification of tracks within a band are written on a disk directly in Gray code without being encoded as is typically done in many prior art systems. This provides only the identification of the track within the band, but not the absolute track address, and accordingly other information must be utilized to identify the absolute track address. In Lewis et al. an erase gap precedes the preamble for each sector to identify where the servo information begins. In Lewis et al. an A BURST and a B BURST are utilized for fine track positioning.
A second prior art approach to embedded servo for rigid disk drives is illustrated in U.S. Pat. No. 4,669,004, issued to Moon et al., May 26, 1987 and entitled "High Capacity Disk File with Embedded Sector Servo". A continuation of Moon et al. U.S. Pat. No. 4,669,004 issued as U.S. Pat. No. 4,783,705 on Nov. 8, 1988, entitled "High Capacity Disk File with Embedded Sector Servo and SCSI Interface". In the above identified Moon et al. patents the track address is written in each of the four burst signals located at different radial positions to eliminate the need for phase coherence for each of the tracks on the disk and the track address is written on a 1,7 recording code. Disadvantages of the servo system illustrated in the above two Moon et al. patents when compared to the invention described and claimed herein are that in Moon et al. a phase locked loop is required to read track head and sector information, and since a phase locked loop is being utilized a long burst (8.8 micro seconds) is required to lock-in the phase of the phase locked loop.